Manufacture op melamine



May 26,v 1942.l

A. C.'WIN TRINGAHAM Al..

MANUFACPURE OF MELAMINE Filed May 2a, 1941 Patented May 26,1942

MANUFACT Andrews C. Wintring, Glen Ridge, and Victor L. King, Bound Brook, N. d., asslors to American Cyanamid N. Y., a corporation of :1

Application May 28, 1941, Serial N o. 395,586 3 Claims. .(Ci. 26th-243) This invention relates to the manufacture oi melamine from cyanamide or dicyandiamide. More' particularly, the invention deals with an improvement in a process for the production of Y melamine of high purity which is described in an ,application of Victor L. King, Serial No. 344,638,'1ed July-10, 1940, and in a modication of that process which, is described in our copending application, Serial No. 380,618, tiled leb-` ruary 26, v1941.

yIn the` application of Victor L. King referredto above there is described a method for the manufacture of melamine in which a mixture of about 1 part of cyanamide or dicyandiamide with from 1t to 2 parts of liquid ammonia is heated in an autoclave', with or without the addition of.A

anhydrous methanol or other diluerits to reduce the. autoclave pressure.- been heated at 100200 C. for about 2-10 hours in thcvautoclave it is discharged directly into a body'of Water, which serves to dissolve out amidines of the type of guanidine and guanylurea that would otherwise contaminate the product. In-ourfcopending application, Serial No. 380,618 wei-'described ia modification of this process in which an 'organic hydroxy compound capable o forming a water-soluble iron salt such as tartaric acid,v glycerol or mannitol is dissolved in the water, into which the autoclave contents are discharged. These compounds prevent the pra1 cipitation of iron by the ammonia contained in theautcclave chargeand thereby avoid contarn-n ination of the melamine product by iron.

The present invention relates to a further im provement in the processes described in the applications referred to above which overcomes the problem of contamination of the melamine by the lubricant vused in operating the autoclave,` g Large-,scale operation ofthe process described in our'earlier applications has demonstrated that thisis another serious source of contamination .of the melamine product, which it is 'a principal object of the present invention to avoid.

n .the accompanying drawing we have illustrated an agitated autoclave of a type that has been'used successfully in preparing melamine by the process referred to above. This autoclave consists generally of a heavy, pressure-resisting shell i with top and bottom heads 2 and held in place by annular clamping rings d and t re spectively.` Proper temperature conditions vi'itln7 in the 'autoclave are maintained by the presence of a series of vertical heating tubes :6, mounted between upper and lower headers i and t which are provided with inlet and outlet pipes t and 55 body of Water,

After the charge has Company, New York,

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It for steam or other suitable heating medium. The autoclave charge' is recirculated over these heating elements by means of an agitator il,

driven through a shaft l2 by means of a motor it and a suitable gear reduction l5.

The agitator shaft l2 is mounted in a bearing l5 and passes through a stumng box i6, which is lubricated by a lubricant admitted through a nipple Il connecting by a line le with a suitablereservoir is. Pressure between the reservoir it and the autoclave is equalized by a pipe 2d, so I that the lubricant flows into the bearing by gravi- A ty under a constant hydrostatic hea When the above-described autoclave was operated in the manner described for the production of melamine we soon discovered that it was necessary to maintain an actual dow of lubricant over the wearing surfaces of the bearing it Y and into the autoclave in order to prevent the deposition of small melamine crystals therein When a hydrocarbon lubricant was used this in,

ltration contaminated the melamine formedin the autoclave, and since the subsequent purication step consisted in washing out water-soluble impurities it was impossible to remove the hydrocarbon oil by this method. By applying the principles of the present invention, we have succeeded in overcoming this source of contamination oi the u. f. and in itsv preferred form our invention actually improves the subsequent purication step.

We have found that .these objects'are accomplished by maintaining a flow of a water-soluble lubricant over the bearing le instead of the hydrocarbon oil that was formerly used. e have found that adequate lubrication is obtained byemploying such compounds as ethylene glycol, glycerine. triethanolamine, Turkey red oil or similar water-soluble compounds of appreciable viscosity as the sole lubricant, and by supplying an adequate ow of a water-soluble material of this class we are able to lubricate the'bearin and prevent the deposition of melamine therein while avoiding completely the presence of a ihydrocarbon oil as a source of contamination ci e me naar process. The ilow of the lubrithe bearing le does not result in permanent contamination of the product, for the water-soluble lubricant is completely removed in the subsequent washing step in which the contents of the autoclave are drownedin a For this reason we are enabled to supply any quantity of lubricant that may be necessary to provide lubrication of the bearing and to prevent the deposition of melamine crystals therein. irrespective of the amounts that may pass into the autoclave. Moreover, when organic hydroxy compounds such as glycol. glycerine, mannitol, polyethylene glycols or mixtures thereof are employed their presence in the autoclave charge actually aids the subsequent washing step, for the hydroxy compound will form a water-soluble complex with any iron that may be present and thereby aid in obtaining an ironfree melamine product.

From the foregoing the process which constitutes our present invention is evident. The manufacture of melamine is carried out in the manner outlined in our two copending applications referred to above, and includes the steps of heating in an autoclave a solution of cyanamide or dicyandiamide in a solvent comprising liquid ammonia while agitating the charge by a rotating agitator and subsequently discharging the contents of the autoclave into a body of water while lubricating the bearing of the agitator shaft by applying thereto a water-soluble lubricant. This lubricant is preferably' a polyhydric alcohol of the type of ethylene glycol,

polyethylene glycol or glycerine. and our pre-- ferred lubricant is glycerine itself. This compound has a surprisingly good lubricating action at temperatures of o-200 C. on the agitator bearings and has given excellent results as the sole lubricant in large-scale commercial practice. Moreover it is readily soluble in water in all proportions, and through the bearing into the autoclave are readily and completely removed from the melamine in the subsequent washing step. Since glycerine will form a water-'solublecomplex with iron compounds its presence in the wash water used in the drowning tank is actually an advantage. The

the quantities that leak use of glycerine as a lubricant therefore improves the operation of the process as a whole both by avoiding contamination of the melamine by the hydrocarbon oils ordinarily used for lubricating purposes and by.aiding in the removal of iron in the subsequent purification step.

What we claim is:

y1'. In the manufacture of melamine by a process which includes the steps of heating in an autoclave a solution of a member of the group consisting of cyanamide and dicyandiamide in a. solvent comprising liquid ammonia while agitating the charge by a rotating agitator and subsequently discharging the contents of the autoclave intol a body of water, the improvement which consists in lubricating the agitator bearing by applying thereto a water-soluble lubricant.

2. In the manufacture of melamine by a process which includes the steps of heating in an autoclave a solution of a member of the group consisting of cyanamide and dicyandiamide in a solvent comprising liquid ammonia while agitating the charge by a rotating agitator and subsequently discharging the contents of the autoclave into a body of water, the improvement which consists in lubricating the agitator bearing by applying thereto a water-soluble lubricant comprising a polyhydrlc alcohol.

3. In the manufacture of melamine by a process which includes the steps of heating in an autoclave a -solutlon of.a member of the group consisting of cyanamide and dicyandiamide in a solvent comprising liquid ammonia while agitating the charge by a rotating agitator and subsequently discharging the contents of the autoclave into a body of water, the improvement which consists in lubricating the agitator bearing by applying thereto a water-soluble lubricant comprising glycerine.

ANDREWS C. WINTRINGHAM.

VICTOR L KING. 

